1. The Field of the Invention
The present invention relates generally to analog circuits. More specifically, the present invention relates to high-voltage, low-offset operational amplifiers with rail-to-rail common mode input range that may be implemented using a digital CMOS process.
2. Background and Relevant Art
Electronic circuitry provides complex functionality that is proving ever more useful. Electronic circuitry pervades our modern lives in areas such as communication, entertainment, travel, productivity, and the like. Complex circuit functionality is enabled by cooperative interaction between a variety of electronic components. Such electronic components may include analog components such as, for example, operational amplifiers, as well as digital circuitry. It is ever more desirable to integrate as many electronic components as possible on a single chip. This reduces the number of discrete components required to accomplish certain functionality thereby providing significant size, cost, and reliability benefits.
However, integrating high voltage components such as high voltage operational amplifiers into a low voltage digital process can be troublesome. To achieve high voltage analog components in a digital process, special devices such as the lateral DMOS or the extended drain MOSFET are constructed. These special devices have higher breakdown voltages to withstand high voltage levels. However, these devices are not very practical for precision analog design because of their poor matching characteristics and device limitations. For example, poor matching characteristics can lead to high offset voltages for operational amplifiers.
FIG. 5 illustrates a conventional operational amplifier 500 that uses two differential pairs to achieve rail-to-rail common mode input range. “Rail-to-Rail” common mode input range means that the common mode input range spans from VSS to VDD, the two supply voltage rails.
Referring to FIG. 5, field effect transistors Q501 and Q502 are part of the PMOS differential input pair that generates a common-mode range from VSS to VDD-VTP, where VTP is the threshold voltage of the PMOS field effect transistor. Hereinafter, field effect transistors may also be referred to simply as “transistors”. Transistors Q503 and Q504 are an NMOS differential input pair that supply a common-mode range from VTN to VDD, where VTN is the threshold voltage of the NMOS field effect transistor. Using these two differential input stages together allows for a rail-to-rail common-mode input range.
Other conventional operational amplifiers that achieve a wide common mode input range include the use of bulk driven transistors, or bulk modulated transistors at the input stage. The bulk driven input devices have a gate to source bias such that they are always on. The bulks are tied to the input signal that effectively modulates the threshold voltage of the input devices and also their current.
These conventional operational amplifiers are generally designed for and effective in low voltage amplifiers. These designs could be modified for high voltage designs by replacing the low voltage devices with high voltage MOS devices. However, there are several drawbacks to this. First, extra processing steps would typically be used over and above standard digital CMOS processes to generate a dual well process to ensure that the amplifier is in a safe operating area. Second, high voltage MOS devices can also have a mismatch between devices that is three to five times larger than the low voltage MOSFET, so the amplifiers will have a higher offset. Third, the high voltage devices also have the limitation that the gate-source and gate-bulk voltages are two to twenty times smaller than the potential that can be applied to the drain-bulk, drain-gate and drain-source nodes. Accordingly, extra care should be taken to modify the design to meet these three constraints.
Other amplifier architectures have been used in the design of high voltage applications. One such architecture uses low voltage devices, but requires a special process that can have devices with differing threshold voltages.
Accordingly, what would be advantageous is a high-voltage rail-to-rail common mode input range operational amplifier that may be implemented using standard digital CMOS processes. It would further be desirable if such an operational amplifier would have low offset voltage for precision applications.